Conventionally, various data conversion methods are proposed as a code conversion method for recording signals composed of digital data by converting them to codes in conformity with the characteristics of a recording system. These recording codes are required to have a good signal-to-noise ratio (S/N ratio) when reproduced and fewer direct current and low frequency components to improve data restoring capability when data is corrected and data transfer characteristics.
For example, several characteristic recording codes are proposed for digital video tape recorders and an NRZ (non return to zero) code and mirror squared code are widely known as typical recording codes.
When the NRZ code is used independently, an amount of direct current and low frequency components may be reduced, and thus it is contemplated that a probability of appearance of a direct current component is restricted to a given value by using a scrambled coding or an amount of the direct current component is reduced by the addition of a redundant component to the bit string of a conversion code.
The applicants have proposed 8 to 11 conversion (a method of converting 8-bit data to 11-bit code), as an example of the scrambling method, in Japanese Patent Appln. No.036696/1992 (filed on Feb. 24, 1992).
However, even if the scrambling method is applied to 8 to 11 conversion, a direct current component cannot be removed because this is a conversion into an odd number of bits. Further, when the method of adding a redundant component to a bit string is applied to an 8 to 14 conversion (a conversion method of converting 8-bit data to 14-bit code), this method actually corresponds to an 8 to 17 conversion because 3 bits are usually added and thus a problem arises in that a conversion efficiency is lowered.
Thus, a block coding for converting data composed of an NRZ code into a n-bit code having fewer direct current and low frequency components (m.ltoreq.n) by a m-bit unit is contemplated.
In the case of, for example, an 8 to 10 conversion, this block coding prepares two conversion tables, i.e., a conversion table with a code weight "+2" and a conversion table with a code weight "-2" with single code data and the conversion tables are changed so that a digital sum variation (DSV) of a code system as a whole becomes to "0".
Here, the code weight is a value obtained by determining the digital sum variation of each code data (that is, a digital sum variation for 10 bits) by assuming that a code "1" and code "0" correspond to a value "+1" and value "-1", respectively; and a digital sum variation of a code system as a whole is a digital sum variation of a specific code weight in the code system as a whole. As a result, it can be assumed that when the digital sum variation DSV of the code system has a limited value, a direct current component is not produced.
Incidentally, even in the aforesaid conversion system in which the two conversion tables are prepared and a conversion code is selected from one of the conversion tables in which a digital sum variation DSV is made smaller for each code data, it cannot be said that a direct current component and error rate are sufficiently lowered; therefore, a conversion method capable of further lowering the direct current component and error rate is desired.
In addition, although the use of the 8 to 11 conversion method by which 8-bit digital data is converted to 11-bit code is examined at present as this type of the modulation coding method, this 8 to 11 conversion method creates difficulty in removing a direct current component from the property thereof because it converts an even number of bits to an odd number of bits, and thus the use of suitable conversion tables is desired.